System and method for preparing liquefied natural gas through double-circulation precooling mixed refrigerant

Abstract

The invention discloses a system and a method for preparing liquefied natural gas through a double-circulation precooling mixed refrigerant, and relates to the technical field of natural gas liquefaction. The double-circulation precooling module of the first precooling refrigerant and the second precooling refrigerant is utilized, a plurality of precooling temperature gradients are provided through multi-stage pressure reduction, the heat transfer temperature difference can be further reduced, the heat transfer efficiency is improved, the precooling temperature value can be further reduced through the circulation module of the second precooling refrigerant, and the heat transfer efficiency is improved. The problem of insufficient cold supply when a traditional single refrigerant participates in circulating pre-cooling is solved, meanwhile, introduction of heavy hydrocarbon is avoided, and the operation stability of the system is improved. In addition, circulating refrigerant compression equipment adopted by the invention is a normal-temperature compressor, so that the equipment investment cost is greatly reduced, that is, under the same operating power, the productivity is greatly increased, and the energy consumption utilization rate is higher.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas liquefaction, and in particular to a system and method for preparing liquefied natural gas by double-circulation precooling mixed refrigerant. Background Art

[0002] The booming development of the liquefied natural gas (LNG) industry has played a significant role in promoting economic growth, optimizing energy structure and reducing emissions. Traditional natural gas liquefaction processes mainly include: cascade liquefaction process, expander liquefaction process and mixed refrigerant liquefaction process. In particular, the mixed-refrigerant cycle (MRC) process has become the mainstream process for domestic natural gas liquefaction due to its simplicity of process, moderate energy consumption, low equipment and investment costs, and easy operation and management.

[0003] The MRC liquefaction process is a refrigeration liquefaction process based on the mixture of some or all components such as C1~C5 and N2 as refrigerants. The mixed refrigerant is pressurized, condensed, cooled, and throttled to achieve refrigeration and liquefaction of natural gas. The main process flows of MRC are as follows: single-stage mixed refrigerant liquefaction process (SMR), mixed refrigerant liquefaction process with propane precooling (C3 / MRC), dual mixed refrigerant liquefaction process (DMR) and mixed refrigerant cascade liquefaction process (MFC).

[0004] At present, about 80% of the base load natural gas liquefaction plants in the world use the C3 / MRC liquefaction process. This process adds a propane pre-cooling step on the basis of single-stage mixed refrigerant liquefaction, effectively reducing the cold loss in the LNG liquefaction process, thereby improving the overall energy efficiency. However, the C3 / MRC liquefaction process still has the following disadvantages:

[0005] (1) The C3 / MRC process usually relies on propane as the sole refrigerant in the precooling stage, and this propane is usually separated from natural gas and contains a small amount of heavy hydrocarbon impurities. When this type of impure propane is throttled, gas-liquid separation will occur, and the gaseous propane has no phase change in the heat exchanger, resulting in poor heat exchange effect.

[0006] (2) The minimum pre-cooling temperature of propane is about -40°C, which is a fixed value and cannot change according to the changes in the external ambient temperature. Therefore, when the composition of the raw natural gas changes and the external ambient temperature changes seasonally, it will affect the stable production of the LNG unit and increase the operating energy consumption. In order to make up for this deficiency and ensure sufficient cooling capacity, heavy hydrocarbon components (C4, C5) are added to the mixed refrigerant in the main cycle to match the pre-cooling cycle module. Due to the high boiling point and freezing point of heavy hydrocarbon components, when the temperature of natural gas liquefaction reaches -162°C, the heavy hydrocarbon components will form solids and cause serious blockage of pipelines and equipment, affecting the normal operation of the main cooling cycle.

[0007] (3) The pre-cooling system of the existing natural gas liquefaction process with pre-cooling mostly adopts a low-temperature compressor refrigeration method. The cost of the low-temperature compressor accounts for about 80% of the total pre-cooling system. The equipment is expensive and the related process design needs to be further optimized.

[0008] In summary, the existing precooling mixed refrigerant liquefaction process still has deficiencies in precooling effect, energy consumption and cost investment utilization, and needs further optimization and improvement. It is urgent to develop a liquefaction process with good precooling effect, low energy consumption and low cost.

[0009] In view of this, the present invention is proposed. Summary of the invention

[0010] The purpose of the present invention is to provide a system and method for preparing liquefied natural gas by double-circulation precooling mixed refrigerant, aiming to provide a liquefaction process with good precooling effect, low energy consumption and low cost.

[0011] The present invention is achieved in that:

[0012] In a first aspect, an embodiment of the present invention provides a system for preparing liquefied natural gas by precooling a mixed refrigerant through a double cycle, comprising a first precooling cycle module, a second precooling cycle module and a mixed refrigerant refrigeration cycle module;

[0013] The first precooling circulation module includes a precooling box, a first precooling compression unit, a first precooling cooler, at least one first precooling refrigerant throttle valve and at least one first evaporative heat exchange unit, the precooling box is provided with a second precooling refrigerant inlet, a second precooling refrigerant outlet, a mixed refrigerant inlet, a mixed refrigerant outlet, at least one first precooling refrigerant inlet and at least one first precooling refrigerant outlet; the precooling box is used to recover the cold of the first precooling refrigerant and precool the second precooling refrigerant, the mixed refrigerant and the natural gas at the same time, the first precooling refrigerant outlet is connected to the first precooling compression unit, the first precooling refrigerant outlet is connected to the first precooling compression unit, and the second precooling refrigerant outlet is connected to the first precooling compression unit. The outlet of a precooling compression unit is connected to the first precooling cooler. The first precooling refrigerant is cooled and depressurized by the first precooling refrigerant throttle valve, and then passes through the first evaporative heat exchange unit for gas-liquid separation, and partially uses the liquid phase of the first precooling refrigerant to exchange heat with the second precooling refrigerant and / or the mixed refrigerant and / or natural gas, and finally all vaporizes to recover latent cooling. The vaporized first precooling refrigerant and the gas-liquid separated gas phase first precooling refrigerant both enter the precooling cold box to provide a cold source to further recover sensible cooling, and then is pressurized by the first precooling compression unit, and then enters the first precooling cooler for cooling, and then enters the precooling cold box for precooling and temperature reduction;

[0014] The second precooling circulation module includes a precooling box, a second precooling compression unit, a second precooling cooler, at least one second precooling refrigerant throttle valve and at least one second evaporative heat exchange unit. The precooling box is provided with a natural gas inlet, a natural gas outlet, a mixed refrigerant inlet, a mixed refrigerant outlet, at least one second precooling refrigerant inlet, and at least one second precooling refrigerant outlet. The precooling box is used to recover the cold of the second precooling refrigerant and precool the mixed refrigerant and natural gas at the same time; the precooling box, the first evaporative heat exchange unit, and the second precooling refrigerant are sequentially passed through ... second pre The second precooling refrigerant of the original precooling is cooled and depressurized by the second precooling refrigerant throttle valve, and then enters the second evaporative heat exchange unit for gas-liquid separation and utilizes part of the liquid-phase second precooling refrigerant for heat exchange with the mixed refrigerant and / or the natural gas output by the precooling cold box to finally be completely vaporized to recover latent cooling. The vaporized second precooling refrigerant and the gas-liquid separated gas-phase second precooling refrigerant both enter the precooling cold box as a cold source to further recover sensible cooling, and then is pressurized by the second precooling compression unit, and then enters the second precooling cooler for cooling, and then enters the precooling cold box for precooling and temperature reduction;

[0015] The mixed refrigerant refrigeration cycle module includes a mixed refrigerant compression unit, a mixed refrigerant cooler, a cryogenic cold box and a mixed refrigerant throttle valve. The mixed refrigerant is pressurized by the mixed refrigerant compression unit and then cooled by the mixed refrigerant cooler. The pressurized mixed refrigerant is pre-cooled by the pre-cooling cold box, the first pre-cooling refrigerant heat exchanger and the second pre-cooling refrigerant heat exchanger in sequence, and the pre-cooled mixed refrigerant enters the cryogenic cold box; a multi-stage throttling process is adopted, the mixed refrigerant is cooled in the cryogenic cold box and then throttled and cooled by the mixed refrigerant throttle valve, and then returns to the cryogenic cold box to exchange heat with natural gas in the cryogenic cold box, gradually vaporizes and provides cooling capacity for the deep cooling zone of the cryogenic cold box; the mixed refrigerant vaporizes and absorbs heat in the cryogenic cold box, and then enters the pre-cooling cold box to exchange heat with the second pre-cooling refrigerant, the mixed refrigerant and natural gas to further recover latent cooling, and after being reheated to room temperature, it returns to the mixed refrigerant compression unit for recycling.

[0016] In an optional embodiment, each first evaporative heat exchange unit includes a first precooling gas-liquid separator and a first precooling refrigerant heat exchanger, and each second evaporative heat exchange unit includes a second precooling gas-liquid separator and a second precooling refrigerant heat exchanger;

[0017] The first precooling refrigerant is cooled and depressurized by the first precooling refrigerant throttle valve, and then passes through the first precooling gas-liquid separator for gas-liquid separation to obtain a liquid-phase first precooling refrigerant and a gas-phase first precooling refrigerant. Part of the liquid-phase first precooling refrigerant is used for heat exchange with the second precooling refrigerant and / or the mixed refrigerant and / or the natural gas in the first precooling refrigerant heat exchanger to recover the liquid-phase first precooling refrigerant latent cooling. After the vaporization, the first precooling refrigerant is separated from the gas-liquid to obtain the gas-phase first precooling refrigerant, which enters the precooling cold box to provide a cold source for further recovery of sensible cooling; another part of the liquid-phase first precooling refrigerant enters the first precooling refrigerant throttle valve of the next stage;

[0018] The second precooling refrigerant which is precooled by the precooling cold box and the first precooling refrigerant heat exchanger in sequence is cooled and reduced in temperature and pressure by the second precooling refrigerant throttle valve, and then enters the second precooling gas-liquid separator for gas-liquid separation to obtain a liquid second precooling refrigerant and a gaseous second precooling refrigerant. Part of the liquid second precooling refrigerant is used for heat exchange with the mixed refrigerant and / or the natural gas output from the precooling cold box in the second precooling refrigerant heat exchanger and is finally completely vaporized to recover latent cooling. After the vaporized second precooling refrigerant, both it and the gaseous second precooling refrigerant enter the precooling cold box as a cold source to further recover sensible cooling, and then enter the second precooling cooler for cooling after being pressurized by the second precooling compression unit, and then enter the precooling cold box for cooling; another part of the liquid second precooling refrigerant enters the second precooling refrigerant throttle valve of the next stage.

[0019] In an optional embodiment, the first precooling circulation module includes at least one flash tank, and each first precooling refrigerant throttle valve is connected to a flash tank or a first precooling gas-liquid separator;

[0020] The bottom liquid phase outlet of each flash tank is connected to the first pre-cooling refrigerant throttle valve of the next stage.

[0021] In an optional embodiment, the flash tank is at least two stages, the first precooling gas-liquid separator is at least two stages, the first precooling refrigerant heat exchanger is at least two stages, and each stage of the first precooling gas-liquid separator corresponds to a first stage of the first precooling refrigerant heat exchanger;

[0022] A first pre-cooling refrigerant throttle valve is provided before each stage of flash tank and each stage of first pre-cooling gas-liquid separator;

[0023] Preferably, the gas phase produced by the first flash tank is returned to the first precooling compression unit, and the gas phase first precooling refrigerant produced by the second flash tank is returned to the precooling cold box as a cold source to recover sensible cooling and then returned to the first precooling compression unit;

[0024] Preferably, the first-stage first pre-cooling refrigerant heat exchanger is used for heat exchange between the second pre-cooling refrigerant and the liquid-phase first pre-cooling refrigerant produced by the first-stage first pre-cooling gas-liquid separator, and the second-stage first pre-cooling refrigerant heat exchanger is used for heat exchange between the second pre-cooling refrigerant and the mixed refrigerant and the liquid-phase first pre-cooling refrigerant produced by the second-stage first pre-cooling gas-liquid separator.

[0025] In an optional embodiment, the first precooling cycle module further includes a first precooling refrigerant reservoir, and the first precooling refrigerant reservoir is located between the first precooling cooler and the first-stage first precooling refrigerant throttle valve.

[0026] In an optional embodiment, the second precooling gas-liquid separator is at least three stages, the second precooling refrigerant heat exchanger is at least three stages, and each stage of the second precooling gas-liquid separator corresponds to a stage of the second precooling refrigerant heat exchanger;

[0027] A first-stage second pre-cooling refrigerant throttle valve is provided before each second-stage second pre-cooling gas-liquid separator;

[0028] Preferably, the gaseous second precooling agent produced by each stage of the second precooling gas-liquid separator enters the precooling cold box as a cold source to further recover its sensible cooling;

[0029] Preferably, the liquid second precooling agent produced by the first-stage second precooling gas-liquid separator is divided into two parts, one part enters the first-stage second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas, and after vaporization, returns to the precooling cold box as a cold source to further recover its sensible cooling, and the other part enters the second-stage second precooling refrigerant throttle valve to reduce temperature and pressure, and then enters the second-stage second precooling gas-liquid separator;

[0030] The liquid second precooling agent produced by the secondary second precooling gas-liquid separator is divided into two parts. One part enters the secondary second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas to vaporize and recover its latent cooling. After vaporization, it returns to the precooling cold box as a cold source to further recover its sensible cooling. The other part of the liquid second precooling agent enters the tertiary second precooling refrigerant throttle valve to reduce temperature and pressure, and then enters the tertiary second precooling gas-liquid separator.

[0031] By analogy, the liquid second precooling agent produced by the last-stage second precooling gas-liquid separator all enters the last-stage second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas for vaporization to recover its latent cooling, and after vaporization, returns to the cryogenic cold box as a cold source to further recover its sensible cooling.

[0032] In an optional embodiment, a second precooling refrigerant reservoir is arranged between the last-stage first precooling refrigerant heat exchanger and the first-stage second precooling refrigerant throttle valve, and the second precooling refrigerant output by the last-stage first precooling refrigerant heat exchanger first enters the second precooling refrigerant reservoir, and then passes through the first-stage second precooling refrigerant throttle valve to reduce temperature and pressure.

[0033] In an optional embodiment, a gas-liquid separation device is provided between the last-stage second pre-cooling refrigerant heat exchanger and the cryogenic cold box. The mixed refrigerant output from the last-stage second pre-cooling refrigerant heat exchanger enters the gas-liquid separation device to produce a liquid mixed refrigerant and a gas mixed refrigerant. Both the liquid mixed refrigerant and the gas mixed refrigerant enter the cryogenic cold box. After being cooled, they are cooled and depressurized by a mixed refrigerant throttle valve and then return to the cryogenic cold box as a cold source.

[0034] In an optional embodiment, it also includes an LNG throttle valve and an LNG storage tank, and the natural gas is cooled by the cryogenic cold box and then decompressed by the LNG throttle valve, and then enters the LNG storage tank;

[0035] Preferably, the gas generated by the LNG storage tank enters the cryogenic cold box and the pre-cooling cold box as a cold source to recover cold energy, and then outputs the BOG product.

[0036] In a second aspect, the present invention provides a method for producing liquefied natural gas by circulating precooling a mixed refrigerant, wherein the system of any one of the aforementioned embodiments is used to produce liquefied natural gas.

[0037] Preferably, the method comprises:

[0038] Natural gas liquefaction process: The purified natural gas is cooled to -70℃~-80℃ in the pre-cooling box, then cooled to -95℃~-100℃ in the second pre-cooling refrigerant heat exchangers at all levels, and then enters the deep cold box to be cooled to -150℃~-170℃ for output;

[0039] The first precooling refrigerant circulation process: the first precooling refrigerant is cooled to 35℃~45℃ by the first precooling compression unit and the first precooling cooler, and then enters the first precooling gas-liquid separator for gas-liquid separation after throttling by the first precooling refrigerant throttle valve to obtain a liquid first precooling refrigerant and a gaseous first precooling refrigerant. A part of the liquid first precooling refrigerant enters the next-level first precooling refrigerant throttle valve; the other part of the liquid first precooling refrigerant is used for heat exchange with the second precooling refrigerant and / or the mixed refrigerant in the first precooling refrigerant heat exchanger, and after vaporization, both the liquid first precooling refrigerant and the gaseous first precooling refrigerant enter the precooling cold box for reheating to 30℃~40℃, and then is cooled to 35℃~45℃ by the first precooling compression unit and the first precooling cooler, forming a closed first precooling refrigerant circulation process;

[0040] The second precooling refrigerant circulation process: the second precooling refrigerant is cooled to -6℃~-7℃ by the precooling cold box, and is cooled to -35℃~-45℃ after passing through the first precooling refrigerant heat exchangers of each stage, and then enters the second precooling gas-liquid separator for gas-liquid separation after throttling by the second precooling refrigerant throttle valve to obtain liquid second precooling refrigerant and gas second precooling refrigerant, and a part of the liquid second precooling refrigerant enters the second precooling refrigerant throttle valve of the next stage; the other part of the liquid second precooling refrigerant is used for heat exchange with the mixed refrigerant and / or natural gas in the second precooling refrigerant heat exchanger, and after vaporization, both the second precooling refrigerant and the gaseous second precooling refrigerant enter the precooling cold box for reheating to 35℃~40℃, and then enters the second precooling compression unit and the second precooling cooler to cool to 38℃~42℃, and then enters the precooling cold box to cool to -6℃~-7℃, forming a closed second precooling refrigerant circulation process;

[0041] Mixed refrigerant circulation process: the mixed refrigerant is cooled to -32℃~-36℃ by the precooling cold box, cooled to -38℃~-42℃ after passing through the first precooling refrigerant heat exchangers of each level, cooled to -95℃~-100℃ after passing through the second precooling refrigerant heat exchangers of each level, cooled to -160℃~-165℃ by the deep cold box, throttled by the mixed refrigerant throttle valve, and returned to the deep cold box as the reflux refrigerant to provide cooling capacity, and then returned to the precooling cold box for cooling capacity recovery after being reheated to -95℃~-100℃, and then returned to the mixed refrigerant compression unit and the mixed refrigerant cooler for compression and cooling after being returned to the precooling cold box to be cooled to -32℃~-36℃, forming a closed mixed refrigerant circulation process;

[0042] Preferably, the first precooling refrigerant and the second precooling refrigerant are independently selected from at least one of Freon, ethane, ethylene, propylene and propane; the mixed refrigerant is selected from at least two of nitrogen, methane, ethylene, ethane, propylene, propane, butane and pentane.

[0043] The present invention has the following beneficial effects: the present invention utilizes a dual-circulation precooling module of a first precooling refrigerant and a second precooling refrigerant, and multiple precooling temperature gradients provided by multi-stage decompression can further reduce the heat transfer temperature difference and improve the heat transfer efficiency, and the circulation module of the second precooling refrigerant can further reduce the precooling temperature value, solving the problem of insufficient cooling capacity when a traditional single refrigerant participates in circulating precooling, while also avoiding the introduction of heavy hydrocarbons and improving the stability of system operation. In addition, the circulating refrigerant compression equipment used in the present invention is a normal temperature compressor, which greatly reduces the equipment investment cost, that is, under the same operating power, the production capacity is greatly increased and the energy consumption utilization rate is higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0045] Figure 1 A flow chart of producing liquefied natural gas by double-cycle precooling mixed refrigerant provided in Example 1;

[0046] Figure 2 A flow chart of producing liquefied natural gas by double-cycle precooling mixed refrigerant provided in Example 2;

[0047] Figure 3 A flow chart of producing liquefied natural gas by double-cycle precooling mixed refrigerant provided in Example 3;

[0048] Figure 4 A flow chart of conventional liquefied natural gas production (SMR refrigeration cycle) provided for Comparative Example 1;

[0049] Figure 5 The flow chart of single-stage double-cycle pre-cooling mixed refrigerant to produce liquefied natural gas is provided for comparative example 2.

[0050] Description of main component symbols:

[0051] 100-first precooling circulation module; 101-precooling cold box; 102-first precooling compression unit; 103-first precooling cooler; 104-first precooling refrigerant storage tank; 105-first precooling refrigerant throttle valve of the first stage; 106-first flash tank; 107-secondary first precooling refrigerant throttle valve; 108-secondary flash tank; 109-third-level first precooling refrigerant throttle valve; 110-first precooling gas-liquid separator of the first stage; 111-first precooling refrigerant heat exchanger of the first stage; 112-fourth-level first precooling refrigerant throttle valve; 113-secondary first precooling gas-liquid separator; 114-secondary first precooling refrigerant heat exchanger; 115-fifth-level first precooling refrigerant throttle valve; 116-third-level first precooling gas-liquid separator; 117-third-level first precooling refrigerant heat exchanger;

[0052] 200-second precooling circulation module; 201-precooling cold box; 202-second precooling compression unit; 203-second precooling cooler; 204-second precooling refrigerant reservoir; 205-first-level second precooling refrigerant throttle valve; 206-first-level second precooling gas-liquid separator; 207-second-level second precooling refrigerant throttle valve; 208-second-level second precooling gas-liquid separator; 209-third-level second precooling refrigerant throttle valve; 210-third-level second precooling gas-liquid separator; 211-first-level second precooling refrigerant heat exchanger; 212-second-level second precooling refrigerant heat exchanger; 213-third-level second precooling refrigerant heat exchanger; 214-fourth-level second precooling refrigerant throttle valve; 215-fourth-level second precooling gas-liquid separator; 216-fourth-level second precooling refrigerant heat exchanger;

[0053] 300-mixed refrigerant refrigeration cycle module; 301-mixed refrigerant compression unit; 302-mixed refrigerant cooler; 303-cryogenic cold box; 304-mixed refrigerant throttle valve; 305-gas-liquid separation device;

[0054] 401-LNG throttle valve; 402-LNG storage tank;

[0055] 001-cold box; 002-mixed refrigerant compression unit; 003-mixed refrigerant cooler; 004-mixed refrigerant separation tank; 005-mixed refrigerant throttle valve; 006-mixed refrigerant throttle valve; 007-LNG throttle valve; 008-LNG storage tank;

[0056] 501-precooling cold box; 502-first precooling compression unit; 503-first precooling cooler; 504-propylene liquid storage tank; 505-first precooling refrigerant throttle valve; 506-first flash tank; 507-second first precooling refrigerant throttle valve; 508-first precooling gas-liquid separator; 509-first precooling refrigerant heat exchanger; 601-precooling cold box; 602-second precooling compression unit; 603-second Precooling cooler; 604-first-stage second precooling refrigerant throttle valve; 605-first-stage flash tank; 606-second-stage precooling refrigerant throttle valve; 607-first-stage second precooling gas-liquid separator; 608-first-stage second precooling refrigerant heat exchanger; 701-mixed refrigerant compression unit; 702-mixed refrigerant cooler; 703-deep cold box; 704-mixed refrigerant throttle valve; 801-LNG throttle valve; 802-LNG storage tank. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0058] like Figure 1 As shown, an embodiment of the present invention provides a dual-cycle pre-cooling system for producing liquefied natural gas using a mixed refrigerant, including a first pre-cooling cycle module 100, a second pre-cooling cycle module 200 and a mixed refrigerant refrigeration cycle module 300. The first pre-cooling cycle module 100 is used to form a cycle of a first pre-cooling refrigerant (such as propylene), the second pre-cooling cycle module 200 is used to form a cycle of a second pre-cooling refrigerant (such as ethylene), and the mixed refrigerant refrigeration cycle module 300 is used to form a cycle of a mixed refrigerant (such as nitrogen, methane, ethylene).

[0059] The first precooling circulation module 100 includes a precooling cold box 101, a first precooling compression unit 102, a first precooling cooler 103, at least one first precooling refrigerant throttle valve, at least one first precooling gas-liquid separator, and at least one first precooling refrigerant heat exchanger. Figure 1 The first precooling gas-liquid separator is a two-stage first precooling gas-liquid separator, including a first-stage first precooling gas-liquid separator 110 and a second-stage first precooling gas-liquid separator 113. Each first-stage first precooling gas-liquid separator corresponds to a first precooling refrigerant heat exchanger, and the two-stage first precooling gas-liquid separators correspond to the first-stage first precooling refrigerant heat exchanger 111 and the second-stage first precooling refrigerant heat exchanger 114, respectively. The first precooling refrigerant throttle valve is as follows Figure 1 The first stage first pre-cooling refrigerant throttle valve 105, the second stage first pre-cooling refrigerant throttle valve 107, the third stage first pre-cooling refrigerant throttle valve 109, and the fourth stage first pre-cooling refrigerant throttle valve 112.

[0060] Furthermore, the precooling cold box 101 is provided with a second precooling refrigerant inlet, a second precooling refrigerant outlet, a mixed refrigerant inlet, a mixed refrigerant outlet, at least one first precooling refrigerant inlet and at least one first precooling refrigerant outlet; the precooling cold box 101 is used to recover the coldness of the first precooling refrigerant and precool the second precooling refrigerant, the mixed refrigerant and the natural gas at the same time, the first precooling refrigerant outlet is connected to the first precooling compression unit 102, the outlet of the first precooling compression unit 102 is connected to the first precooling cooler 103, and the first precooling refrigerant is connected to the first precooling compression unit 102. After the refrigerant throttling valve cools and reduces the pressure, it passes through the first pre-cooling gas-liquid separator for gas-liquid separation and utilizes part of the liquid phase of the first pre-cooling refrigerant to exchange heat with the second pre-cooling refrigerant and / or the mixed refrigerant and / or the natural gas, and finally all of them are vaporized to recover latent cooling. The vaporized first pre-cooling refrigerant and the gas-liquid separated gas phase first pre-cooling refrigerant then enter the pre-cooling cold box 101 to provide a cold source to further recover sensible cooling, and then are pressurized by the first pre-cooling compression unit 102, and then enter the first pre-cooling cooler 103 for cooling, and then enter the pre-cooling cold box 101 for pre-cooling and cooling, and the above process is repeated.

[0061] In some embodiments, the first precooling cycle module 100 includes at least one flash tank (e.g. Figure 1 The first flash tank 106 and the second flash tank 108 in the process are connected to a flash tank or a first precooling gas-liquid separator after each first precooling refrigerant throttle valve; the bottom liquid phase outlet of each flash tank is connected to the first precooling refrigerant throttle valve of the next stage. The flash tank may not be set, and it can be adjusted according to the process conditions.

[0062] There is no limit to the number of flash tanks, which can be two or more stages; there is no limit to the number of first pre-cooling gas-liquid separators, which can be two or more stages; each stage of the first pre-cooling gas-liquid separator corresponds to a first pre-cooling refrigerant heat exchanger, and a first pre-cooling refrigerant throttle valve is arranged before each stage of the flash tank and each stage of the first pre-cooling gas-liquid separator.

[0063] Figure 1The first stage flash tank and the second stage pre-cooling gas-liquid separator are two-stage flash tanks and two-stage first pre-cooling gas-liquid separators. The first stage first pre-cooling refrigerant throttle valve 105 is throttled and then enters the first stage flash tank 106. The high-pressure gas phase produced by the first stage flash tank 106 returns to the first pre-cooling compression unit 102. The liquid phase produced by the first stage flash tank 106 is throttled by the second stage first pre-cooling refrigerant throttle valve 107 and then enters the second stage flash tank 108; the gas phase first pre-cooling refrigerant produced by the second stage flash tank 108 returns to the pre-cooling cold box 101 as a cold source to recover the sensible cooling and then returns to the first pre-cooling compression unit. The liquid phase produced by the second stage flash tank 108 is throttled by the third stage first pre-cooling refrigerant throttle valve 109 and then enters the first stage first pre-cooling gas-liquid separator 110 for gas-liquid separation; the first stage first pre-cooling gas The liquid phase produced by the liquid separator 110 is divided into two parts. One part enters the first precooling refrigerant heat exchanger 111 of the first stage to exchange heat with the second precooling refrigerant (such as ethylene). After vaporization, it flows parallel to the gas phase produced by the first precooling gas-liquid separator 110 and returns to the precooling cold box 101 as a cold source. The other part enters the first precooling refrigerant throttle valve 112 of the fourth stage to throttle and then enters the second first precooling gas-liquid separator 113 for gas-liquid separation. The liquid phase produced by the second first precooling gas-liquid separator 113 all enters the second first precooling refrigerant heat exchanger 114 to exchange heat with the second precooling refrigerant and the mixed refrigerant. After vaporization, it flows parallel to the gas phase produced by the second first precooling gas-liquid separator 113 and returns to the precooling cold box 101 as a cold source.

[0064] In some embodiments, the first precooling cycle module 100 also includes a first precooling refrigerant reservoir 104, which is located between the first precooling cooler 103 and the first-level first precooling refrigerant throttle valve 105, and is used to temporarily store the first precooling refrigerant.

[0065] Specifically, the first precooling compression unit 102 may be a compressor unit, and similarly, the first precooling cooler 103 may also be a group of coolers, so as to achieve compression and cooling of multiple streams of materials.

[0066] The second precooling cycle module 200 includes a precooling box 201, a second precooling compression unit 202, a second precooling cooler 203, at least one second precooling refrigerant throttle valve (such as Figure 1 The first-stage second precooling refrigerant throttle valve 205, the second-stage second precooling refrigerant throttle valve 207, the third-stage second precooling refrigerant throttle valve 209, and at least one second precooling gas-liquid separator (such as Figure 1The first-stage second precooling gas-liquid separator 206, the second-stage second precooling gas-liquid separator 208, the third-stage second precooling gas-liquid separator 210) and at least one second precooling refrigerant heat exchanger (the first-stage second precooling refrigerant heat exchanger 211, the second-stage second precooling refrigerant heat exchanger 212, the third-stage second precooling refrigerant heat exchanger 213). Each stage includes a second precooling refrigerant throttle valve, a second precooling gas-liquid separator, and a second precooling refrigerant heat exchanger, and the number of the three is set according to the number of stages of the process.

[0067] The second precooling refrigerant (such as ethylene) output by the precooling box 201 is compressed by the second precooling compression unit 202, and then cooled by the second precooling cooler 203, and then cooled by the precooling box 101 and the first precooling refrigerant heat exchangers of each stage in turn, and then cooled and reduced in pressure by the second precooling refrigerant throttle valve, and then enters the second precooling gas-liquid separator for gas-liquid separation to obtain a liquid second precooling agent and a gas second precooling agent. Part of the liquid second precooling agent is used for heat exchange with the mixed refrigerant and / or the natural gas output by the precooling box 201 in the second precooling refrigerant heat exchanger, and after vaporization, it enters the precooling box 201 in parallel with the gas second precooling agent for reheating, and then enters the second precooling cooler 203 for cooling after being pressurized by the second precooling compression unit 202, and then enters the precooling box 101 for cooling, completing the circulation of the second precooling refrigerant; another part of the liquid second precooling agent enters the second precooling refrigerant throttle valve of the next stage.

[0068] The pre-cooling cold box 201 is provided with a natural gas inlet, a natural gas outlet, a mixed refrigerant inlet, a mixed refrigerant outlet, at least one second pre-cooling refrigerant inlet, and at least one second pre-cooling refrigerant outlet. The pre-cooling cold box 201 is used to recover the cold of the second pre-cooling refrigerant and pre-cool the mixed refrigerant and natural gas at the same time, thereby playing a role in initially cooling the natural gas.

[0069] In some embodiments, the second precooling gas-liquid separator may be at least three stages, the second precooling refrigerant heat exchanger may be at least three stages, and each stage of the second precooling gas-liquid separator corresponds to a stage of the second precooling refrigerant heat exchanger. Figure 1 In the three-stage case, a second precooling refrigerant throttle valve is arranged before each second precooling gas-liquid separator, and the gaseous second precooling refrigerant produced by each second precooling gas-liquid separator enters the precooling cold box 201 as a cold source to further recover its sensible cooling.

[0070] It should be noted that the liquid second precooling agent produced by the first-level second precooling gas-liquid separator 206 is divided into two parts, one part enters the first-level second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas, and after vaporization, returns to the precooling cold box 201 as a cold source to further recover its sensible cooling, and the other part enters the second-level second precooling refrigerant throttle valve 207 to cool down and reduce pressure, and then enters the second-level second precooling gas-liquid separator 208; similarly, the liquid second precooling agent produced by the second-level second precooling gas-liquid separator 208 is divided into two parts, one part enters the second-level second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas, and after vaporization, returns to the precooling cold box 201 as a cold source to further recover its sensible cooling, and the other part enters the second-level second precooling refrigerant throttle valve 207 to cool down and reduce pressure, and then enters the second-level second precooling gas-liquid separator 208. The mixed refrigerant and / or natural gas is heat exchanged and vaporized to recover its latent cooling, and after vaporization, it returns to the precooling cold box 201 as a cold source to further recover its sensible cooling, and another part of the liquid-phase second precooling agent enters the third-stage second precooling refrigerant throttle valve 209 to reduce temperature and pressure, and then enters the third-stage second precooling gas-liquid separator 210; and so on, the liquid-phase second precooling agent produced by the last-stage second precooling gas-liquid separator all enters the last-stage second precooling refrigerant heat exchanger to heat exchange with the mixed refrigerant and / or natural gas to recover its latent cooling, and after vaporization, it returns to the precooling cold box 201 as a cold source to further recover its sensible cooling.

[0071] In some embodiments, in the last stage first pre-cooling refrigerant heat exchanger (such as Figure 1 A second precooling refrigerant reservoir 204 is provided between the secondary first precooling refrigerant heat exchanger 114) and the primary second precooling refrigerant throttle valve 205, and the second precooling refrigerant outputted by the last stage first precooling refrigerant heat exchanger first enters the second precooling refrigerant reservoir 204 for temporary storage, and then is cooled and depressurized by the primary second precooling refrigerant throttle valve 205. The second precooling refrigerant reservoir 204 may be provided or not provided according to process requirements.

[0072] The mixed refrigerant refrigeration cycle module 300 includes a mixed refrigerant compression unit 301, a mixed refrigerant cooler 302, a cryogenic cold box 303 and a mixed refrigerant throttle valve 304. The mixed refrigerant is pressurized by the mixed refrigerant compression unit 301 and then cooled by the mixed refrigerant cooler 302. The pressurized mixed refrigerant is precooled by the precooling cold box 101, the first precooling refrigerant heat exchangers of each stage, and the second precooling refrigerant heat exchangers of each stage in sequence, and the precooled mixed refrigerant enters the cryogenic cold box 303. A multi-stage throttling process is adopted, and the mixed refrigerant is cooled by throttling by the mixed refrigerant throttle valve 304 after cooling in the cryogenic cold box 303, and then returns to the cryogenic cold box 303 to exchange heat with natural gas in the cryogenic cold box 303, and gradually vaporizes to provide the cryogenic cold box 303 with cold capacity in the cryogenic zone. After absorbing heat by vaporizing in the cryogenic cold box 303, the mixed refrigerant enters the pre-cooling cold box 101 to exchange heat with the second pre-cooling refrigerant, the mixed refrigerant and natural gas to further recover latent cooling. After being reheated to room temperature, it returns to the mixed refrigerant compression unit 301 for recycling, thus completing the circulating refrigeration of the mixed refrigerant.

[0073] In some embodiments, an LNG throttle valve 401 and an LNG storage tank 402 are also included. After the natural gas is cooled by the cryogenic cold box 303, it is decompressed by the LNG throttle valve 401 and then stored in the LNG storage tank 402. The gas generated by the LNG storage tank 402 enters the cryogenic cold box 303 and the pre-cooling cold box 201 as a cold source to recover cold energy, and then outputs the BOG product.

[0074] like Figure 2 As shown, a further separation step can be added to the first precooling cycle module 100. Figure 1 On this basis, a five-stage first precooling refrigerant throttle valve 115, a three-stage first precooling gas-liquid separator 116, and a three-stage first precooling refrigerant heat exchanger 117 are added. The liquid phase produced by the two-stage first precooling gas-liquid separator 113 is divided into two parts, one part enters the two-stage first precooling refrigerant heat exchanger 114 for heat exchange; the other part enters the five-stage first precooling refrigerant throttle valve 115 for throttling, and then enters the three-stage first precooling gas-liquid separator 116 for gas-liquid separation, and all the produced liquid phases enter the three-stage first precooling refrigerant heat exchanger 117 for heat exchange with the mixed refrigerant and the second precooling refrigerant, and after vaporization, it returns to the precooling cold box 101 in parallel with the gas phase obtained from the three-stage first precooling gas-liquid separator 116.

[0075] Similarly, Figure 2 A fourth-stage second precooling refrigerant throttle valve 214, a fourth-stage second precooling gas-liquid separator 215, and a fourth-stage second precooling refrigerant heat exchanger 216 are also added. The liquid phase produced by the third-stage second precooling gas-liquid separator 210 is divided into two parts, one part enters the third-stage second precooling refrigerant heat exchanger 213 for heat exchange with natural gas and mixed refrigerant, and the other part enters the fourth-stage second precooling gas-liquid separator 215 for gas-liquid separation after throttling by the fourth-stage second precooling refrigerant throttle valve 214; all the liquid phase produced by the fourth-stage second precooling gas-liquid separator 215 enters the fourth-stage second precooling refrigerant heat exchanger 216 for heat exchange with natural gas and mixed refrigerant, and after vaporization, it flows in parallel with the gas phase produced by the fourth-stage second precooling gas-liquid separator 215 and enters the precooling cold box 201 as a cold source.

[0076] In addition, the gas-liquid separator and heat exchanger of the evaporative heat exchange unit can also be replaced by a single evaporator, that is, multiple first evaporative heat exchange units and multiple second evaporative heat exchange units are set; the streams passing through each stage of the evaporative heat exchange unit can be specifically set according to the actual application, including but not limited to: ① The mixed refrigerant is all pre-cooled through the multi-stage evaporative heat exchange unit of the second pre-cooling cycle process, and the natural gas can be set to pass through a specific number of evaporative heat exchange stages (one, two or three and above) according to the actual application; ② The natural gas is all pre-cooled through the multi-stage evaporative heat exchange unit of the "second pre-cooling cycle process", and the mixed refrigerant can be set to pass through a specific number of evaporative heat exchange stages (one, two or three and above) according to the actual application.

[0077] In some embodiments, the flash tank may also be equipped with a heat exchanger for cold recovery, and the gas-liquid separator and heat exchanger of the evaporative heat exchange unit may be replaced by a single evaporator; the streams passing through each stage of the evaporative heat exchange unit may be specifically set according to the actual application, including but not limited to: ① The second precooling refrigerant is all precooled through the multi-stage evaporative heat exchange unit of the first precooling cycle process, and the mixed refrigerant can be set according to the actual application The specific number of evaporative heat exchange stages (one, two or three and above) passed by the mixed refrigerant is set, and the natural gas is not precooled through the first precooling module; ② The second precooling refrigerant is all Precooling is carried out through the multi-stage evaporative heat exchange unit of the first precooling cycle process. The natural gas can be set to pass through a specific number of evaporative heat exchange stages (one, two or three and above) according to actual applications, and the mixed refrigerant is not precooled through the first precooling module; ③ The second precooling refrigerant is all precooled through the multi-stage evaporative heat exchange unit of the first precooling cycle process, and the mixed refrigerant and natural gas are both precooled through the multi-stage evaporative heat exchange unit of the first precooling cycle process, and the two can each be set according to the actual application. The specific number of evaporative heat exchange stages (one, two or three and above) passed.

[0078] In some embodiments, Figure 3 As shown, a gas-liquid separation device 305 is arranged between the last-stage second pre-cooling refrigerant heat exchanger and the cryogenic cold box 303. The mixed refrigerant output by the last-stage second pre-cooling refrigerant heat exchanger enters the gas-liquid separation device 305 to produce a liquid mixed refrigerant and a gas mixed refrigerant. Both the liquid mixed refrigerant and the gas mixed refrigerant enter the cryogenic cold box 303. After being cooled, they are cooled and depressurized by the mixed refrigerant throttle valve 304, and then return to the cryogenic cold box 303 as a cold source. Figure 3 It can be two-stage throttling of the mixed refrigerant, or it can be multi-stage throttling, and the specific throttling stages can be set according to the actual application.

[0079] It should be noted that the dual-cycle pre-cooling compressor used in the embodiment of the present invention can be single-stage compression or multi-stage compression (not limited to two-stage, three-stage, four-stage, etc.) according to the specific application situation; the pre-cooling evaporative heat exchange unit used in the embodiment of the present invention can be single-stage evaporation or multi-stage evaporation (not limited to two-stage, three-stage, four-stage, etc.) according to the specific application situation; the mixed refrigerant compression cycle module involved in the embodiment of the present invention can have a single-stage throttling or multi-stage throttling (not limited to two-stage, three-stage, four-stage, etc.) in the cold box, and the mixed refrigerant entering the cold box can be a gas-phase refrigerant or a gas-phase refrigerant + liquid-phase refrigerant. The pre-cooling compressor and the mixed refrigerant compressor involved in the above-mentioned present invention can both use normal temperature compressors (including reciprocating, screw or centrifugal compressors, etc.).

[0080] It should be noted that the precooling box provided in the embodiment of the present invention has many flow channels. In order to avoid confusion caused by too many interweaving of flows in the drawings, it is divided into two identical devices (i.e., precooling box 101 and precooling box 201) in the drawings, both of which represent precooling boxes. In other words, precooling box 101 and precooling box 201 represent a total precooling box.

[0081] The embodiment of the present invention also provides a method for producing liquefied natural gas by circulating precooling mixed refrigerant, and the system of any one of the above embodiments is used to produce liquefied natural gas. The method has the advantages of low energy consumption, wide application range, and can be suitable for large, medium and small natural gas liquefaction.

[0082] It should be noted that, on the one hand, compared with the conventional cascade liquefaction process, the mixed refrigerant liquefaction process with a precooling module involved in the present invention can reduce the temperature of the produced LNG to below -162°C, and the system device produces less BOG flash gas; on the other hand, compared with the conventional single refrigerant circulation precooling technology, the natural gas liquefaction system provided by the present invention utilizes a dual-circulation precooling module based on C2 and C3, and multiple precooling temperature gradients provided by multi-stage decompression, which can further reduce the heat transfer temperature difference and improve the heat transfer efficiency, and the C2 precooling circulation module can further reduce the precooling temperature value, which solves the problem of insufficient cold capacity when the traditional single refrigerant participates in the circulation precooling, and also avoids the introduction of heavy hydrocarbons, and improves the stability of the system operation. In addition, the circulating refrigerant compression equipment used in the present invention is a normal temperature compressor, which greatly reduces the equipment investment cost, that is, under the same operating power, the production capacity is greatly increased, and the energy consumption utilization rate is higher.

[0083] In some embodiments, the method for producing liquefied natural gas by circulating precooling mixed refrigerant provided by the embodiments of the present invention comprises the following steps:

[0084] Natural gas liquefaction process: the purified natural gas is cooled to -70℃~-80℃ (such as -75.8℃) by the pre-cooling cold box 201, and then cooled to -95℃~-100℃ (such as -97℃) by the second pre-cooling refrigerant heat exchangers at various levels, and then enters the deep cooling cold box 303 to be cooled to -150℃~-170℃ (such as -162℃) for output; finally, it is formed into LNG product through the LNG throttle valve 401 and transported to the LNG storage tank 402, and the generated BOG is returned to the pre-cooling cold box for cold recovery.

[0085] The first precooling refrigerant circulation process: the first precooling refrigerant is cooled to 35°C~45°C by the first precooling compression unit 102 and the first precooling cooler 103, and then enters the first precooling gas-liquid separator for gas-liquid separation after throttling by the first precooling refrigerant throttle valve to obtain a liquid-phase first precooling refrigerant and a gas-phase first precooling refrigerant. A part of the liquid-phase first precooling refrigerant enters the next-level first precooling refrigerant throttle valve; the other part of the liquid-phase first precooling refrigerant is used for heat exchange with the second precooling refrigerant and / or the mixed refrigerant and / or the natural gas in the first precooling refrigerant heat exchanger to recover the liquid-phase first precooling refrigerant latent cooling. The gas-phase first precooling refrigerant obtained by vaporization and gas-liquid separation enters the precooling cold box 101 for reheating to 30°C~40°C, and then is cooled to 35°C~45°C by the first precooling compression unit 102 and the first precooling cooler 103, forming a closed first precooling refrigerant circulation process;

[0086] Second precooling refrigerant circulation process: the second precooling refrigerant is cooled to -6℃~-7℃ through the precooling cold box 101, and is cooled to -35℃~-45℃ after passing through the first precooling refrigerant heat exchangers of each stage, and then enters the second precooling gas-liquid separator for gas-liquid separation after throttling by the second precooling refrigerant throttle valve to obtain liquid second precooling refrigerant and gas second precooling refrigerant, and a part of the liquid second precooling refrigerant enters the second precooling refrigerant throttle valve of the next stage; the other part of the liquid second precooling refrigerant is used for heat exchange with the mixed refrigerant and / or natural gas in the second precooling refrigerant heat exchanger and is finally all vaporized to recover latent cooling, and after vaporization, both the second precooling refrigerant and the gaseous precooling refrigerant enter the precooling cold box 201 for reheating to 35℃~40℃, and then enters the second precooling compression unit 202 and the second precooling cooler 203 to cool to 38℃~42℃, and then enters the precooling cold box 101 to cool to -6℃~-7℃, forming a closed second precooling refrigerant circulation process;

[0087] Mixed refrigerant circulation process: the mixed refrigerant is cooled to -32°C to -36°C by the precooling cold box 101, cooled to -38°C to -42°C after passing through the first precooling refrigerant heat exchangers of each stage, cooled to -95°C to -100°C after passing through the second precooling refrigerant heat exchangers of each stage, cooled to -160°C to -165°C by the deep cold box 303, throttled by the mixed refrigerant throttle valve 304, and returned to the deep cold box 303 as a reflux refrigerant to provide cooling capacity, and then returned to the precooling cold box 201 for cooling capacity recovery after being reheated to -95°C to -100°C, and then returned to the mixed refrigerant compression unit 301 and the mixed refrigerant cooler 302 for compression and cooling after being reheated to 35°C to 40°C, and then returned to the precooling cold box 101 to be cooled to -32°C to -36°C, forming a closed mixed refrigerant circulation process;

[0088] In some embodiments, the first precooling refrigerant and the second precooling refrigerant are independently selected from at least one of Freon, ethane, ethylene, propylene and propane, and can be any one or more of the above, such as propylene + propane, propylene + ethylene, etc. The mixed refrigerant is selected from at least two of nitrogen, methane, ethylene, ethane, propylene, propane, butane and pentane, and can be a mixture of two or more of the above.

[0089] In some embodiments, the purified natural gas can also be pre-cooled by the first pre-cooling circulation module 100 and the second pre-cooling circulation module 200 before being sent to the cryogenic cold box 303 for LNG production. In this way, the natural gas only needs to be cooled to a lower temperature in the pre-cooling cold box. By setting more levels of pre-cooling temperature gradients, the heat transfer temperature difference between the cold and hot sides can be further reduced. damage.

[0090] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0091] Example 1

[0092] This embodiment provides a method for producing liquefied natural gas by circulating precooling mixed refrigerant, such as Figure 1 As shown, the specific steps are as follows:

[0093] Natural gas liquefaction process: The purified natural gas first enters the pre-cooling cold box 201 to be cooled to -75.8℃, then enters the secondary second pre-cooling refrigerant heat exchanger 212 to be cooled to -85℃, then passes through the tertiary second pre-cooling refrigerant heat exchanger 213 to be cooled to -97℃, then enters the 25 deep cold box 303 to be cooled to -162℃, and finally passes through the LNG throttle valve 401 to form LNG product and transported to the LNG storage tank 402, and the generated BOG returns to the pre-cooling cold box for cold recovery.

[0094] The first precooling cycle process: propylene is compressed and cooled to 40°C by the first precooling compression unit 102 and the first precooling cooler 103, and then sent to the propylene liquid storage tank, and then the propylene enters the first flash tank 106 for gas-liquid separation through the first precooling refrigerant throttle valve 105, and the high-pressure gas phase (pressure is 1.105MPaA) propylene separated by the first flash tank 106 directly returns to the fourth-stage suction tank of the first precooling compression unit 102, and the liquid phase propylene separated by the first flash tank 106 enters the second flash tank 108 after throttling by the second-stage first precooling refrigerant throttle valve 107; the medium-pressure gas phase (pressure is 0.539MPaA) propylene separated by the second flash tank 108 is recovered by the precooling cold box 101 and reheated to 37°C, and then enters the third-stage suction tank of the first precooling compression unit 102, and the liquid phase propylene separated by the second flash tank 108 enters the first-stage first precooling gas-liquid separator after throttling by the third-stage first precooling refrigerant throttle valve 109. 110, the liquid phase propylene produced by the first-stage evaporation of propylene is divided into two streams, one of which provides cold for the ethylene in the first-stage first precooling refrigerant heat exchanger 111, and after being vaporized, it returns to the precooling cold box 101 together with the low-pressure gas phase (pressure is 0.259MPaA) propylene produced by the first-stage evaporation separation for cold recovery, and enters the second-stage suction tank of the first precooling compression unit 102 after the low-pressure gas phase propylene is reheated to 36.4°C; the other liquid phase propylene enters the second-stage first precooling gas-liquid separator 113 after throttling by the fourth-stage first precooling refrigerant throttle valve 112, and the liquid phase propylene produced by the second-stage evaporation of propylene provides cold for the ethylene and mixed refrigerant in the second-stage first precooling refrigerant heat exchanger 114, and after being vaporized, it returns to the precooling cold box 101 together with the low-low-pressure gas phase (pressure is 0.124MPaA) propylene produced by the second-stage evaporation separation for cold recovery, and enters the first-stage suction tank of the first precooling compression unit 102 after the low-low-pressure gas phase propylene is reheated to 37°C. The high-pressure, medium-pressure, low-pressure and low-low-pressure propylene after the pre-cooling cold box 101 recovers the cold energy is compressed and cooled in the first pre-cooling compression unit 102 and the first pre-cooling cooler 103 respectively, and finally the propylene is sent to the propylene liquid storage tank after being cooled to 40°C, thus forming a closed propylene refrigeration cycle process.

[0095] The second precooling cycle process: ethylene first enters the precooling cold box 101 and is cooled to -6.5°C, then enters the first-level precooling refrigerant heat exchanger 111 and is cooled to -22°C, and then passes through the second-level first precooling refrigerant heat exchanger 114 and is cooled to -40°C before being sent to the ethylene liquid storage tank, and then the ethylene enters the first-level second precooling gas-liquid separator 206 through the first-level second precooling refrigerant throttle valve 205. The liquid phase ethylene produced by the first-level evaporation of ethylene is divided into two streams, one of which enters the first-level second precooling refrigerant heat exchanger 211 to provide cooling for the mixed refrigerant, and after vaporization, it returns to the precooling cold box 201 together with the high-pressure gas phase (pressure is 0.534MPaA) ethylene produced by the first-level evaporation separation for cooling recovery, and after the high-pressure gas phase ethylene is reheated to 37°C, it enters the third-level suction tank of the second precooling compression unit 202; the other liquid phase ethylene enters the second-level second precooling gas-liquid separator 208 after throttling by the second-level second precooling refrigerant throttle valve 207, and the ethylene The liquid propylene produced by the secondary evaporation of ethylene is divided into two streams, one of which enters the secondary second precooling refrigerant heat exchanger 212 to provide cooling for the mixed refrigerant and natural gas, and then is vaporized and returned to the precooling cold box 201 together with the medium-pressure gas phase (pressure is 0.258MPaA) ethylene produced by the secondary evaporation separation for cooling. After the medium-pressure gas phase ethylene is reheated to 37° C., it enters the secondary suction tank of the second precooling compression unit 202; the other liquid ethylene enters the tertiary second precooling gas-liquid separator 210 after throttling by the tertiary second precooling refrigerant throttle valve 209, and the liquid ethylene produced by the tertiary evaporation of ethylene enters the tertiary second precooling refrigerant heat exchanger 213 to provide cooling for the mixed refrigerant and natural gas, and then is vaporized and returned to the precooling cold box 201 together with the low-pressure gas phase (pressure is 0.128MPaA) ethylene produced by the tertiary evaporation separation for cooling. After the low-pressure gas phase ethylene is reheated to 37° C., it enters the primary suction tank of the second precooling compression unit 202. The high-pressure, medium-pressure and low-pressure ethylene after the cold is recovered by the pre-cooling cold box 201 are compressed and cooled in the second pre-cooling compression unit 202 and the second pre-cooling cooler 203 respectively. Finally, after the ethylene is cooled to 40°C, it is sent to the pre-cooling cold box 101 to be cooled to -6.5°C, thus forming a closed ethylene refrigeration cycle process.

[0096] Mixed refrigerant refrigeration cycle process: The mixed refrigerant first enters the pre-cooling cold box 101 and is cooled to -34°C, then enters the second-stage first pre-cooling refrigerant heat exchanger 114 and is cooled to -40°C, then enters the first-stage second pre-cooling refrigerant heat exchanger 211 and is cooled to -67°C, then enters the second-stage second pre-cooling refrigerant heat exchanger 212 and is cooled to -85°C, then enters the third-stage second pre-cooling refrigerant heat exchanger 213 and is cooled to -97°C, then enters the deep cold box 303 and is cooled to -162°C, and then passes through the mixed After throttling by the refrigerant throttling valve 304, the refrigerant returns to the deep cold box 303 as the reflux refrigerant to provide cooling for the natural gas liquefaction process, and then is reheated to -97°C, and finally returns to the pre-cooling cold box 201 for cooling recovery. After being reheated to 37°C, it leaves the pre-cooling cold box and enters the first-stage suction tank of the mixed refrigerant compression unit 301. After being compressed and cooled by the mixed refrigerant compression unit 301 and the mixed refrigerant cooler 302, it returns to the pre-cooling cold box 101 and is cooled to -34°C, thus forming a closed mixed refrigerant refrigeration cycle process.

[0097] After testing: the unit energy consumption index of this embodiment is 0.230kW / Nm 3 Purified natural gas is more energy-efficient than conventional SMR refrigeration cycle systems (energy consumption index: 0.322kW / Nm 3 Purified natural gas, see comparative example 1 for details) is about 28.8% less than the single-stage double-cycle pre-cooling mixed refrigerant refrigeration cycle system (energy consumption index: 0.276kW / Nm 3 Purified natural gas, see comparative example 2 for details) is about 17.0% less, and energy consumption is significantly reduced.

[0098] Comparative Example 1

[0099] like Figure 4 As shown, this comparative example provides a conventional liquefied natural gas process (SMR refrigeration cycle system).

[0100] Natural gas liquefaction process: The purified natural gas (about 40°C) is directly cooled to -162°C through cold box 001 and then output. It is then transformed into LNG product through LNG throttle valve 007 and transported to LNG storage tank 008. The generated BOG enters the BOG recovery unit.

[0101] Mixed refrigerant circulation process: The reheated mixed refrigerant (38°C) coming out of the cold box 001 first enters the primary suction tank of the mixed refrigerant compression unit 002, and the mixed refrigerant (40°C) compressed and cooled by the mixed refrigerant compression unit 002 and the mixed refrigerant cooler 003 is sent to the mixed refrigerant separation tank 004 for gas-liquid separation; the high-pressure gas phase (3.19MPaA, 40°C) mixed refrigerant separated by the mixed refrigerant separation tank 004 first enters the cold box 001 to be cooled to -162°C, and then throttled and cooled by the mixed refrigerant throttle valve 005 and returned to the cold box as a high-pressure gas phase return refrigerant Box 001 provides cooling capacity for the deep cooling section of natural gas and mixed refrigerant; the high-pressure liquid phase (3.19MPaA, 40℃) mixed refrigerant separated by the mixed refrigerant separation tank 004 enters the cold box 001 through another flow channel and is cooled to -50℃, and then returns to the cold box 001 as a high-pressure liquid countercurrent refrigerant after throttling and cooling through the mixed refrigerant throttle valve 006, providing cooling capacity for the intermediate cooling section of natural gas and mixed refrigerant. The two streams of reflux side mixed refrigerants are reheated to 38℃ and then discharged from the cold box together, and then return to the first-stage suction tank of the mixed refrigerant compression unit 002, thus forming a closed mixed refrigerant refrigeration cycle process.

[0102] Comparative Example 1 does not have a pre-cooling cycle module. Compared with the liquefaction process with a pre-cooling cycle module, the energy efficiency utilization rate of the mixed refrigerant refrigeration process alone is low, and the cooling loss in the heat exchange cooling process is large. According to preliminary estimates, adding a pre-cooling cycle can provide about 1 / 3 of the cooling capacity for the entire natural gas liquefaction process. On the contrary, not having a cold cycle module will increase the cooling load of the mixed refrigerant and increase energy consumption.

[0103] Comparative Example 2

[0104] like Figure 5 As shown, this comparative example provides a single-stage double-cycle pre-cooling mixed refrigerant refrigeration cycle system.

[0105] Natural gas liquefaction process: The purified natural gas first enters the pre-cooling cold box 601 to be cooled to -76°C, then enters the first-level second pre-cooling refrigerant heat exchanger 608 to be cooled to -98°C, and then enters the deep-freezing cold box 703 to be cooled to -162°C. Finally, it is transformed into LNG product through the LNG throttle valve 801 and transported to the LNG storage tank 802. The generated BOG enters the BOG recovery unit.

[0106] The first precooling cycle process: propylene is compressed and cooled to 40°C by the first precooling compression unit 502 and the first precooling cooler 503, and then sent to the propylene liquid storage tank 504. Then, the propylene enters the first flash tank 506 for gas-liquid separation through the first-stage first precooling refrigerant throttle valve 505. The high-pressure gas phase (pressure is 1.105MPaA) propylene separated by the first flash tank 506 is directly returned to the fourth-stage suction tank of the first precooling compression unit 502. The high-pressure liquid phase propylene (1.105MPaA) separated by the first flash tank 506 is throttled by the second-stage first precooling refrigerant throttle valve 507 and enters the first-stage first precooling gas-liquid separator 508. The low-pressure liquid propylene (0.539 MPaA) produced by the first-stage evaporation provides cooling for the ethylene in the first-stage precooling refrigerant heat exchanger 509, and then returns to the precooling cold box 501 for cooling recovery after being vaporized; the low-pressure gaseous propylene (0.539 MPaA) produced by the first-stage evaporation separation of propylene is also returned to the precooling cold box 501 for cooling recovery; the low-pressure propylene is reheated to 36.7°C and exits the precooling cold box 501, and then enters the first-stage suction tank of the first precooling compression unit 502, and is compressed and cooled to 40°C by the first precooling compression unit 502 and the first precooling cooler 503, and then is sent to the propylene liquid storage tank 504, thus forming a closed propylene refrigeration cycle process.

[0107] Second precooling cycle process: ethylene first enters the precooling cold box 501 and is cooled to -17.5°C, then enters the first precooling refrigerant heat exchanger 509 and is cooled to -41°C, then enters the first flash tank 605 for gas-liquid separation after being throttled by the first second precooling refrigerant throttle valve 604, the medium-pressure gas phase (pressure is 0.543MPaA) ethylene separated by the first flash tank 605 enters the precooling cold box 601, the liquid phase ethylene separated by the first flash tank 605 enters the first second precooling gas-liquid separator 607 for gas-liquid separation after being throttled by the second precooling refrigerant throttle valve 606, and the low-pressure gas generated by the first evaporation of ethylene is 0.543MPaA. The high-pressure liquid ethylene (0.128MPaA) enters the first-stage second precooling refrigerant heat exchanger 608 to provide cooling for the mixed refrigerant and natural gas, and after vaporization, it enters the precooling cold box 601 together with the low-pressure gaseous ethylene (0.128MPaA) produced by the first-stage evaporation of ethylene for cooling recovery; the ethylene is reheated to 37°C and then leaves the precooling cold box 601, and then enters the first-stage suction tank of the second precooling compression unit 602, and then is sent to the second precooling compression unit 602 and the second precooling cooler 603 for compression and cooling to 40°C, and then is sent to the precooling cold box 501 to be cooled to -17.5°C, thus forming a closed ethylene refrigeration cycle process.

[0108] Mixed refrigerant circulation process: the mixed refrigerant first enters the pre-cooling cold box 501 to be cooled to -36°C, then enters the first pre-cooling refrigerant heat exchanger 509 to be cooled to -41°C, then enters the second pre-cooling refrigerant heat exchanger 608 to be cooled to -98°C, and then enters the deep cold box 703 to be cooled to -162°C, and then returns to the deep cold box 703 as a reflux refrigerant after throttling by the mixed refrigerant throttle valve 704 to provide deep cold section cold capacity for the natural gas liquefaction process, and then reheats to about -100°C, and then enters the pre-cooling cold box 601 for cold capacity recovery. After the mixed refrigerant on the reflux side is reheated to 37°C, it leaves the pre-cooling cold box and enters the first suction tank of the mixed refrigerant compression unit 701. After being compressed and cooled by the mixed refrigerant compression unit 701 and the mixed refrigerant cooler 702, it returns to the pre-cooling cold box 501 to be cooled to -36°C, thus forming a closed mixed refrigerant refrigeration cycle process.

[0109] This comparative example 2 is provided with a single-stage double-circulation precooling module, and its energy efficiency utilization rate is greatly improved compared with the mixed refrigerant liquefied natural gas process without a precooling circulation module. However, compared with the mixed refrigerant liquefied natural gas process with a multi-stage double-circulation precooling module, its energy consumption utilization rate is still insufficient.

[0110] In the precooling cycle, the more heat exchange stages there are, the higher the equipment investment and the higher the energy consumption utilization rate. The present invention has found in practice that generally three or four stages of precooling heat exchange can obtain a higher utilization rate, and multi-stage throttling in the mixed refrigerant cycle (deep cooling stage) can also obtain a higher energy consumption utilization rate and reduce production costs.

[0111] The present invention provides a system and method for preparing liquefied natural gas by double-circulation precooling mixed refrigerant, which has the following advantages over conventional liquefied natural gas processes:

[0112] (1) Compared with the conventional cascade liquefaction process, the mixed refrigerant liquefaction process with a precooling module involved in the present invention can reduce the temperature of the produced LNG to below -162°C, and the system device generates less BOG flash gas.

[0113] (2) Compared with the conventional single-cycle precooling mixed refrigerant liquefaction process, the present invention sets up a double-cycle precooling module and adopts a multi-stage precooling evaporator to form a larger number of precooling temperature gradients with lower temperatures, further reducing the heat transfer temperature difference between the cold and hot sides and loss; and a pre-cooling cold box is also set to recover the cold capacity of the low-temperature gas phase pre-cooling refrigerant of each stage of the evaporator (the gas phase refrigerant will be discharged from the cold box after being reheated to room temperature), which makes the energy utilization of the system more efficient, provides sufficient cooling capacity and avoids the introduction of heavy hydrocarbons, thereby improving the stability of system operation.

[0114] (3) The circulating refrigerant compression equipment used in the present invention is a normal temperature compressor. The refrigerant compressor and its associated suction tank do not need to consider the low-temperature resistant material, which can greatly reduce the equipment investment cost. That is, at the same cost and operating power, the production capacity is greatly increased and the energy consumption utilization rate is higher.

[0115] (4) The present invention is provided with a pre-cooling refrigerant flash tank and a multi-stage pre-cooling refrigerant evaporator, which can obtain different levels of evaporation pressures, which are returned to the inlet of each stage of the compressor respectively. Compared with the conventional single-cycle pre-cooling mixed refrigerant liquefaction process with only one evaporation pressure, the energy consumption of the compressor is significantly reduced.

[0116] (5) The pre-cooling refrigerant evaporation heat exchanger of the present invention may be in various forms such as a kettle evaporator, a coiled tube heat exchanger, a plate-fin heat exchanger, etc.

[0117] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A system for preparing liquefied natural gas by precooling mixed refrigerant through double circulation, characterized in that: It includes a first precooling cycle module, a second precooling cycle module and a mixed refrigerant refrigeration cycle module; The first precooling circulation module comprises a precooling box, a first precooling compression unit, a first precooling cooler, at least one first precooling refrigerant throttle valve and at least one first evaporative heat exchange unit, the precooling box is provided with a second precooling refrigerant inlet, a second precooling refrigerant outlet, a mixed refrigerant inlet, a mixed refrigerant outlet, at least one first precooling refrigerant inlet and at least one first precooling refrigerant outlet; the precooling box is used to recover the cold of the first precooling refrigerant and precool the second precooling refrigerant, the mixed refrigerant and the natural gas at the same time, the first precooling refrigerant outlet is connected to the first precooling compression unit, and the first precooling refrigerant outlet is connected to the first precooling compression unit. The outlet of the cold compression unit is connected to the first precooling cooler. After the first precooling refrigerant is cooled and depressurized by the first precooling refrigerant throttle valve, it passes through the first evaporative heat exchange unit for gas-liquid separation and utilizes part of the liquid phase of the first precooling refrigerant to exchange heat with the second precooling refrigerant and / or the mixed refrigerant and / or the natural gas, and finally all of them are vaporized to recover latent cooling. The vaporized first precooling refrigerant and the gas-liquid separated gas phase first precooling refrigerant both enter the precooling cold box to provide a cold source to further recover sensible cooling, and then are pressurized by the first precooling compression unit, and then enter the first precooling cooler for cooling, and then enter the precooling cold box for precooling and temperature reduction; The second precooling circulation module comprises a precooling box, a second precooling compression unit, a second precooling cooler, at least one second precooling refrigerant throttle valve and at least one second evaporative heat exchange unit. The precooling box is provided with a natural gas inlet, a natural gas outlet, a mixed refrigerant inlet, a mixed refrigerant outlet, at least one second precooling refrigerant inlet, and at least one second precooling refrigerant outlet. The precooling box is used to recover the cold of the second precooling refrigerant and precool the mixed refrigerant and natural gas at the same time; the precooling box, the first evaporative heat exchange unit, and the precooling The second pre-cooling refrigerant is cooled and depressurized by the second pre-cooling refrigerant throttle valve, and then enters the second evaporative heat exchange unit for gas-liquid separation and utilizes part of the liquid-phase second pre-cooling refrigerant for heat exchange with the mixed refrigerant and / or the natural gas output by the pre-cooling cold box to finally be completely vaporized to recover latent cooling. The vaporized second pre-cooling refrigerant and the gas-liquid separated gas-phase second pre-cooling refrigerant both enter the pre-cooling cold box as a cold source to further recover sensible cooling, and then is pressurized by the second pre-cooling compression unit, and then enters the second pre-cooling cooler for cooling, and then enters the pre-cooling cold box for pre-cooling and temperature reduction; The mixed refrigerant refrigeration cycle module comprises a mixed refrigerant compression unit, a mixed refrigerant cooler, a cryogenic cold box and a mixed refrigerant throttle valve. The mixed refrigerant is pressurized by the mixed refrigerant compression unit and then cooled by the mixed refrigerant cooler. The pressurized mixed refrigerant is precooled by the precooling cold box, the first precooling refrigerant heat exchanger and the second precooling refrigerant heat exchanger in sequence. The precooled mixed refrigerant enters the cryogenic cold box. A multi-stage throttling process is adopted, and the mixed refrigerant is cooled in the cryogenic cold box and then throttled and cooled by the mixed refrigerant throttling valve, and then returns to the cryogenic cold box to exchange heat with the natural gas in the cryogenic cold box, and gradually vaporizes to provide cooling capacity for the cryogenic zone of the cryogenic cold box; after vaporizing and absorbing heat in the cryogenic cold box, the mixed refrigerant enters the pre-cooling cold box to exchange heat with the second pre-cooling refrigerant, the mixed refrigerant and the natural gas to further recover latent cooling, and after being reheated to room temperature, it returns to the mixed refrigerant compression unit for recycling.

2. The system according to claim 1, characterized in that Each of the first evaporative heat exchange units includes a first precooling gas-liquid separator and a first precooling refrigerant heat exchanger, and each of the second evaporative heat exchange units includes a second precooling gas-liquid separator and a second precooling refrigerant heat exchanger; After the first precooling refrigerant is cooled and depressurized by the first precooling refrigerant throttle valve, the first precooling refrigerant is separated into gas and liquid by the first precooling gas-liquid separator to obtain a liquid-phase first precooling refrigerant and a gas-phase first precooling refrigerant. Part of the liquid-phase first precooling refrigerant is used for heat exchange with the second precooling refrigerant and / or the mixed refrigerant and / or the natural gas in the first precooling refrigerant heat exchanger to recover the liquid-phase first precooling refrigerant latent cooling. After the vaporization, the first precooling refrigerant and the gas-liquid separation to obtain the gas-phase first precooling refrigerant both enter the precooling cold box to provide a cold source for further recovering sensible cooling; another part of the liquid-phase first precooling refrigerant enters the first precooling refrigerant throttle valve of the next stage; The second precooling refrigerant which is precooled by the precooling cold box and the first precooling refrigerant heat exchanger in sequence is cooled and reduced in temperature and pressure by the second precooling refrigerant throttle valve, and then enters the second precooling gas-liquid separator for gas-liquid separation to obtain a liquid second precooling refrigerant and a gaseous second precooling refrigerant. Part of the liquid second precooling refrigerant is used for heat exchange with the mixed refrigerant and / or the natural gas output from the precooling cold box in the second precooling refrigerant heat exchanger and is finally completely vaporized to recover latent cooling. After the vaporized second precooling refrigerant, both the vaporized second precooling refrigerant and the gaseous second precooling refrigerant enter the precooling cold box as a cold source to further recover sensible cooling, and then enters the second precooling cooler for cooling after being pressurized by the second precooling compression unit, and then enters the precooling cold box for cooling; another part of the liquid second precooling refrigerant enters the second precooling refrigerant throttle valve of the next level.

3. The system according to claim 2, characterized in that The first precooling circulation module comprises at least one flash tank, and each of the first precooling refrigerant throttle valves is connected to a flash tank or a first precooling gas-liquid separator; The bottom liquid phase outlet of each flash tank is connected to the first pre-cooling refrigerant throttle valve of the next stage.

4. The system according to claim 3, characterized in that The flash tank has at least two stages, the first pre-cooling gas-liquid separator has at least two stages, the first pre-cooling refrigerant heat exchanger has at least two stages, and each stage of the first pre-cooling gas-liquid separator corresponds to one stage of the first pre-cooling refrigerant heat exchanger; The first pre-cooling refrigerant throttle valve is provided before each stage of the flash tank and each stage of the first pre-cooling gas-liquid separator; Preferably, the gas phase produced by the first flash tank is returned to the first precooling compression unit, and the gas phase first precooling refrigerant produced by the second flash tank is returned to the precooling cold box as a cold source to recover sensible cooling and then returned to the first precooling compression unit; Preferably, the first-stage first pre-cooling refrigerant heat exchanger is used for heat exchange between the second pre-cooling refrigerant and the liquid-phase first pre-cooling refrigerant produced by the first-stage first pre-cooling gas-liquid separator, and the second-stage first pre-cooling refrigerant heat exchanger is used for heat exchange between the second pre-cooling refrigerant and the mixed refrigerant and the liquid-phase first pre-cooling refrigerant produced by the second-stage first pre-cooling gas-liquid separator.

5. The system according to claim 4, characterized in that The first precooling circulation module further includes a first precooling refrigerant reservoir, which is located between the first precooling cooler and a first-stage first precooling refrigerant throttle valve.

6. The system according to claim 2, characterized in that The second pre-cooling gas-liquid separator has at least three stages, the second pre-cooling refrigerant heat exchanger has at least three stages, and each stage of the second pre-cooling gas-liquid separator corresponds to one stage of the second pre-cooling refrigerant heat exchanger; A second pre-cooling refrigerant throttle valve is provided before each stage of the second pre-cooling gas-liquid separator; Preferably, the gaseous second precooling agent produced by the second precooling gas-liquid separator at each stage enters the precooling cold box as a cold source to further recover its sensible cooling; Preferably, the liquid second precooling agent produced by the first-stage second precooling gas-liquid separator is divided into two parts, one part enters the first-stage second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas, and after vaporization, returns to the precooling cold box as a cold source to further recover its sensible cooling, and the other part enters the second-stage second precooling refrigerant throttle valve to reduce temperature and pressure, and then enters the second-stage second precooling gas-liquid separator; The liquid second precooling agent produced by the secondary second precooling gas-liquid separator is divided into two parts, one part enters the secondary second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas for vaporization to recover its latent cooling, and after vaporization, returns to the precooling cold box as a cold source to further recover its sensible cooling, and the other part of the liquid second precooling agent enters the tertiary second precooling refrigerant throttle valve to reduce temperature and pressure, and then enters the tertiary second precooling gas-liquid separator; By analogy, the liquid second precooling agent produced by the last-stage second precooling gas-liquid separator all enters the last-stage second precooling refrigerant heat exchanger to exchange heat with the mixed refrigerant and / or natural gas for vaporization to recover its latent cooling, and after vaporization, returns to the cryogenic cold box as a cold source to further recover its sensible cooling.

7. The system according to claim 6, characterized in that A second precooling refrigerant reservoir is arranged between the last-stage first precooling refrigerant heat exchanger and the first-stage second precooling refrigerant throttle valve. The second precooling refrigerant output by the last-stage first precooling refrigerant heat exchanger first enters the second precooling refrigerant reservoir, and then passes through the first-stage second precooling refrigerant throttle valve for cooling and reducing pressure.

8. The system according to claim 6, characterized in that A gas-liquid separation device is arranged between the last-stage second pre-cooling refrigerant heat exchanger and the cryogenic cold box. The mixed refrigerant output by the last-stage second pre-cooling refrigerant heat exchanger enters the gas-liquid separation device to produce liquid-phase mixed refrigerant and gas-phase mixed refrigerant. Both the liquid-phase mixed refrigerant and the gas-phase mixed refrigerant enter the cryogenic cold box. After being cooled, both are cooled and depressurized by the mixed refrigerant throttle valve, and then return to the cryogenic cold box as a cold source.

9. The system according to claim 1, characterized in that It also includes an LNG throttle valve and an LNG storage tank. After the natural gas is cooled by the cryogenic cold box, it is decompressed by the LNG throttle valve and then enters the LNG storage tank; Preferably, the gas generated by the LNG storage tank enters the deep cooling box and the pre-cooling box as a cold source to recover cold energy, and then outputs the BOG product.

10. A method for producing liquefied natural gas by circulating precooling mixed refrigerant, characterized in that: Producing liquefied natural gas using the system described in any one of claims 1 to 9; Preferably, the method comprises: Natural gas liquefaction process: the purified natural gas is cooled to -70℃~-80℃ by the pre-cooling box, then cooled to -95℃~-100℃ by the second pre-cooling refrigerant heat exchangers at various levels, and then enters the deep cooling box to be cooled to -150℃~-170℃ for output; The first precooling refrigerant circulation process: the first precooling refrigerant is cooled to 35°C~45°C by the first precooling compression unit and the first precooling cooler, and then enters the first precooling gas-liquid separator for gas-liquid separation after throttling by the first precooling refrigerant throttle valve to obtain a liquid first precooling refrigerant and a gaseous first precooling refrigerant. A part of the liquid first precooling refrigerant enters the first precooling refrigerant throttle valve of the next level; the other part of the liquid first precooling refrigerant is used for heat exchange with the second precooling refrigerant and / or the mixed refrigerant in the first precooling refrigerant heat exchanger, and after vaporization, both the liquid first precooling refrigerant and the gaseous first precooling refrigerant enter the precooling cold box for reheating to 30°C~40°C, and then is cooled to 35°C~45°C by the first precooling compression unit and the first precooling cooler, forming a closed first precooling refrigerant circulation process; The second precooling refrigerant circulation process: the second precooling refrigerant is cooled to -6℃~-7℃ by the precooling cold box, cooled to -35℃~-45℃ after passing through the first precooling refrigerant heat exchangers of each stage, and then enters the second precooling gas-liquid separator for gas-liquid separation after throttling by the second precooling refrigerant throttle valve to obtain liquid second precooling refrigerant and gas second precooling refrigerant, and a part of the liquid second precooling refrigerant enters the second precooling refrigerant throttle valve of the next stage; the other part of the liquid second precooling refrigerant is used for heat exchange with the mixed refrigerant and / or the natural gas in the second precooling refrigerant heat exchanger, and after vaporization, both the second precooling refrigerant and the gaseous second precooling refrigerant enter the precooling cold box for reheating to 35℃~40℃, and then enters the precooling cold box for cooling to 38℃~42℃, and then enters the precooling cold box to cool to -6℃~-7℃, forming a closed second precooling refrigerant circulation process; Mixed refrigerant circulation process: the mixed refrigerant is cooled to -32°C to -36°C by the precooling cold box, cooled to -38°C to -42°C after passing through the first precooling refrigerant heat exchangers of each stage, cooled to -95°C to -100°C after passing through the second precooling refrigerant heat exchangers of each stage, cooled to -160°C to -165°C by the deep cold box, throttled by the mixed refrigerant throttle valve, returned to the deep cold box as a reflux refrigerant to provide cooling capacity, reheated to -95°C to -100°C, and returned to the precooling cold box for cooling capacity recovery, reheated to 35°C to 40°C, entered the mixed refrigerant compression unit and the mixed refrigerant cooler for compression and cooling, and then returned to the precooling cold box to be cooled to -32°C to -36°C, forming a closed mixed refrigerant circulation process; Preferably, the first pre-cooling refrigerant and the second pre-cooling refrigerant are independently selected from at least one of Freon, ethane, ethylene, propylene and propane; the mixed refrigerant is selected from at least two of nitrogen, methane, ethylene, ethane, propylene, propane, butane and pentane.

Images